Gas phase shifting multistage displacer cryocooler

ABSTRACT

A displacer cryocooler including a multistage heat exchanger is modified with a phase shifting device connected between stage volumes for gas phase shifting of gas pressure in the cryocooler for improving the efficiency and temperature range of expander cryocoolers.

REFERENCE TO RELATED APPLICATION

The present application is related to applicant's copending applicationsentitled Gas Phase Shifting Multistage Displacer Cryocooler, Ser. No.xx/xxx,xxx, filed xx/xx/xx, Controlled and Variable Gas Phase ShiftingCryocooler Ser. No. yyy/yyy,yyy, filed yy/yy/yy, and Gas Phase ShiftingInertance Gap Pulse Tube Cryocooler, Ser. No. zz/zzz,zzz, filedzz/zz/zz, by the same inventors.

FIELD OF THE INVENTION

The invention relates to the field of refrigeration systems. Moreparticularly, the invention relates to cryocoolers providing phaseshifting of gas pressures using controlled gas phase shifting devices ina multiple stage expander cryocooler and multiple stage pulse tubecryocooler for improved energy transfer and cooling efficiencies.

BACKGROUND OF THE INVENTION

Cryocoolers are mechanical machines used for cooling, heating, andthermal transfer. The cryocoolers typically have multiple internalvolumes for heating and cooling. Multistage coolers are coolers withmore than one cooling or heating stage having more than one volume.Mechanical cryocoolers can be classified according to the type of heatexchangers used, that is, regenerative versus recuperative. Regenerativemechanical cryocoolers can be further classified according to thepresence or absence of valves, thermal compressors, or mechanicalcompressors, and the presence or absence of displacers, such as pulsetubes. Multistage cryocoolers are routinely used for reachingtemperatures below what a single stage cryocooler can achieve. Thestaging of cryocoolers can be done in parallel or in series.

Relevant cryocoolers include displacer cryocoolers and pulse tubecryocoolers. A displacer cryocooler is generally comprised of acompressor connected to an expander. The expander may be a multistageheat exchanger. The compressor and expander are connected together withor without transfer tubes. A pulse tube cryocooler includes a stationaryregenerator connected to a pulse tube that includes a long inertancetube.

The displacer cryocooler is driven by a compressor, which sends apressure wave to the displacer. In a multistage displacer cryocooler,the first stage of the expander pre-cools the gas that enters the secondstage, and the second stage pre-cools the gas that enters the thirdstage, and so on. The cooling capacity at each stage is directlyproportional to the swept volume of the expansion space. Because thecross-sectional areas of the expansion spaces are fixed in a givenmultistage mechanical cryocooler design, the ratio of heat loads amongthe stages that the cryocooler can cool is also fixed. Limited shiftingof loads can be achieved by changing the frequency, charge pressure, ortemperature at each stage.

The efficient pulse tube cryocooler consists of a long inertance tubethat may be up to several meters in length between the warm end of thepulse tube and a buffer volume. Pulse tube coolers are reliableprimarily because pulse tube coolers do not have any cold moving parts,or displacers, or valves that can break. A compressor drives a pistonthat provides the energy needed for the refrigeration. As the pistoncompresses, a parcel of warm gas travels through the regenerator. Thegas is cooled by the matrix of the regenerator, that is, the heatexchanger. Part of the heat from compression Q_(o) is removed at ambienttemperature. The gas is then expanded through the pulse tube and theorifice into the buffer or reservoir volume. Expansion provides coolingQ_(c) that takes place at temperature T_(c). Because the pulse tube doesnot have a displacer, phase shifting is accomplished by a combination ofthe orifice and the buffer volume. Theoretically, maximum coolingefficiency is accomplished with the pressure wave in phase with respectto mass flow at the cold end of the cryocooler. The performance of theorifice pulse tube can be enhanced by incorporating double inlets orbypasses. Moreover, in replacing the orifice with a long capillary knownas the inertance tube, the performance of the cooler can be improved byavoiding irreversible losses associated with a sharp edged orifice. Witha sharp edged orifice, the only variable parameter is the diameter ofthe orifice, limiting heat exchange control. With an inertance tube,there are two variables consisting of the length and diameter of thetube. The phase shift mechanism can be modeled using an LRC circuitanalogy. Inductance L=4L_(t)/(πd_(t) ²) is analog to a flow inertance.Resistance R is analogous to flow impedance. Capacitance C=Mv_(t)/(γRT)is analog to the fluid heat capacity. The tube geometry can be definedwhere L_(t) is the gap length, d_(t) and v_(t) are the length, diameterand internal volume of the inertance tube, η is viscosity, ρ is density,and γ is the specific heat ratio, T is the temperature, and M is themolecular weight. The long inertance tube is used to optimize the gasphase shift. Unfortunately, the long tube geometry is also cumbersome,heavy, and bulky. The long inertance tube does not readily provide themeans for optimizing the gas phase shift between the compressor andreservoir for various applications, which requires different coolingcapacity. The tuning of the gas phase shift is only by setting thelength and diameter of the inertance tube. Accordingly, there are twoways to add inductance, that is, the analogous inertance. One way to addinertance is by increasing the length L_(t) or by decreasing thediameter d_(t). Because resistance is inversely proportional to thefourth power of diameter d_(t), decreasing d_(t) will increaseresistance substantially. Additionally, inertance can be increased byincreasing the length of tube, but this disadvantageously results in along and slender tube.

Pulse tube systems with inertance tubes have been studied boththeoretically and experimentally. The flow circuit in a pulse tube isanalogous to that of an electrical circuit. The optimum phase shiftingcan be obtained by introducing an inertance term into the circuit,instead of relying on a pure resistance circuit caused by a pressuredrop in a sharp edged orifice used for phase shifting. The pulse tubesystem includes an exchanger stage coupled to a pulse tube that iscoupled to a reservoir. The pulse tube system disadvantageously includesan elongated inertance tube having a length that is longer than a meterin most applications. The packaging of a long tube presents problems aswell as in applications where vibration, such as during a launch, maycause failure of the cooler. US Patent Publication No. 20050022539teaches incorporating a hybrid cryocooler with a first stage expanderand a second stage pulse tube design. Because the pulse tube cryocooleruses an orifice or inertance tube between the pulse tube and the surgevolume to optimize cooling, the hybrid design provides an extraparameter for load shifting. Expander cryocoolers are in general moreefficient than pulse tube cryocoolers, however, the displacercryocoolers are less reliable due to the presence of a moving displacer.The hybrid design tends to combine the main disadvantage of a expandercryocooler with that of a pulse tube cryocooler.

Mechanical cryocoolers are used extensively for cooling purposes.Multistage coolers are generally used to reach temperatures below 35° K.The shifting of loads between stages is not flexible in a multistagemechanical cryocooler. Thus, there is disadvantageously a limited rangeof temperature that each stage can achieve relative to other stages.These and other disadvantages are solved or reduced using thisinvention.

SUMMARY OF THE INVENTION

An object of the invention is to provide gas phase shifting in amechanical device.

Another object of the invention is to provide controlled gas phaseshifting using a flow impedance device in a cryocooler.

Yet another object of the invention is to provide controlled gas phaseshifting using a flow inertia device in a cryocooler.

Also another object of the invention is to provide gas phase shiftingusing an inertance gap in a pulse tube cryocooler.

Still another object of the invention is to provide gas phase shiftingusing an impedance device in a multistage displacer cryocooler.

Furthermore, another object of the invention is to provide gas phaseshifting using an inertance device in a multistage displacer cryocooler.

The invention is directed to gas phase shifting in cryocoolers forimproved cooling efficiency. In a first aspect, a valve is disposedbetween stage volumes in a multistage displacer cryocooler. In a secondaspect, inertance gaps are disposed in line of a pulse tube cryocooler.A gas phase shifting means is installed between different volumes of thestages of the cryocooler allowing the cryocooler to operate with widerheat loads and temperature ranges. The gas phase shifting deviceprovides for load shifting between the stages in a multistage mechanicalcryocooler. In a displacer cryocooler, the gas phase shifting can beachieved by installing a phase shifting device between the expansionvolumes. The gas phase shifting device can be a flow impedance device,for example, a valve, a sharp-edged orifice, or a porous medium in anexpander cryocooler. The gas phase shifting device can also be a flowinertia device, for example, an inertance gap in a pulse tube of acryocooler. The gas phase shifting device phase shift the gas pressurein phase with mass flow at the cold end of the cryocooler for maximumcooling. These and other advantages will become more apparent from thefollowing detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a two-stage gas phase shifting cryocooler.

FIG. 2 is a diagram of a pulse tube gas phase shifting inertance gapcryocooler.

FIG. 3A is a diagram of a concentric ring inertance gap.

FIG. 3B is a diagram of a parallel plate inertance gap.

FIG. 4A is a diagram of a motor controlled variable inertance gap.

FIG. 4B is a diagram of a heater controlled variable inertance gap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, a two-stage gas phase shifting valve cryocooler is a modifiedversion of a conventional two-stage cooler with a moving porous pistonheat exchanger. The porous piston heat exchanger, or simply theregenerator, functions as a heat exchanger. The porous piston isbifurcated for heat exchange into a first stage heat exchanger forexchange heat with a first stage volume and into a second stage heatexchanger for heat exchange with a second stage volume. A compressorprovides the pressure and volume work required for cooling, by movingthe piston. The compressor injects gas through an intake conduit into anempty volume, known as a plenum. The gas is cooled when passing throughthe porous heat exchanger. The piston in the cryocooler moves when aparcel of gas is admitted into the plenum through the intake conduit.The parcel of gas then passes through the first stage porous piston heatexchanger being a first stage regenerator into the first stage volume,known as an expander volume. When passing through the piston, the gasexchanges heat with the matrix of the porous piston and is cooled in afirst stage regenerator. The gas is cooled further as the expanderincreases in volume. Part of the admitted gas continues to pass throughthe second stage porous piston heat exchanger for further cooling intothe second stage volume that is also an expander volume for furthercooling. When passing through the second stage heat exchanger, the gasexchanges heat with the matrix of the heat exchanger and is furthercooled. The motion of the porous piston heat exchanger is drivenpneumatically by the compressor or separately by a motor, not shown. Themotion of the porous piston heat exchanger is driven with a phase lagrelative to the compressor motion. When the compressor compresses, gasis admitted into the plenum. As the porous piston heat exchanger movestowards the plenum, expansion takes place at the first and second stageexpander volumes, resulting in cooling. The compression and expansion ofthe first and second stage volumes are synchronized. Thus, thetemperature of the second stage of the cooler is influenced by thetemperature of the first stage, and vice versa.

A controlled gas phase shifting device can be a controlled valve ororifice opening, that is a resistance device, or a controlled tube orgap geometry, that is an inertance device. For maximum cooling, the gaspressure is in phase with mass flow at the cold end of the cryocooler.The gas mass flow of gas moving from the first stage to the second stageis in phase with the pressure difference between the first stage and thesecond stage. The gas phase shifting devices mentioned above arecontrolled by a controller that controls gas flow through a first stageconduit connected to the first stage volume and a second stage conduitconnected to the second stage volume. The valve controls partial gasflows between the first and second expander volumes. The partial gasflows create a phase shift in gas pressures and mass flow between thefirst and second volumes. The valve or orifice resistance device, or atube or gap inertance device disposed between the first stage conduitand second stage conduit, functions as a gas phase shifting device. Byintroducing this phase shifting device between the first and secondstage volumes, a wider temperature range of one stage relative to theother can be achieved. The phase shifting device can be a gas resistancedevice such as valves and orifices, or an inertance device such as tubesand gaps. Different settings of valve opening and orifice size forresistance devices, and diameter, length, width, and thickness of gaps,or the diameter and the length of the tube for inertance can be used tooptimize cooler performance. While shown for two stages, one or morephase shifting devices can be applied to any number of expander volumesin including those having more than two stages and respective volumes.

Referring to FIG. 2, a pulse tube cooler is characterized as having apulse tube coupled between a stationary first stage heat exchanger and areservoir. A compressor drives gas through the first stage heatexchanger or regenerator. Four tubes are used to couple in line thecompressor to the heat exchanger, to the pulse tube, to the inertancegap, and finally to the reservoir. The reservoir is coupled to aninertance gap through a first tube. The inertance gap is coupled to apulse tube through a second tube. The pulse tube is connected to a heatexchanger through a third tube. The heat exchanger is coupled to acompressor through a fourth tube. The compressor includes a pistondriven by a load. The compressor provides the gas pressure work requiredto achieve cooling. The heat exchanger and third tube can be considereda first stage and the pulse tube and the inertance gap can be considereda second stage. As such, the third tube defines an entrance volume as afirst stage volume. The entrance of the fourth tube is a hot end and theexit of the third tube is a cold end of the first stage. The reservoiris an exit volume as a second stage volume.

When the piston in the compressor compresses, a parcel of gas isadmitted into the regenerating heat exchanger through the intakeconduit, that is, the fourth tube. As the gas passes through the heatexchanger, the gas exchanges heat with the matrix, not shown, within theheat exchanger and is cooled. The cooled gas is then passed to the pulsetube that is an empty tube. As the gas moves from the third tube endinto the pulse tube and then to the reservoir through the inertance gap,expansion of the gas occurs resulting in cooling of the gas in the thirdtube. There is created pressure gas phase lag between the compression ofthe gas at the compressor and expansion of the gas through the inertancegap to the reservoir for optimal cooling. The inertance gap is used inplace of an inertance tube. Variously configured inertance gaps can beused.

Referring to FIGS. 3A, 3B, 4A, and 4B in sequence, the inertance gap canbe made of concentric elongated rings. The gap thickness between therings, the diameter of the rings, and the length of the rings, definethe gap geometry, and hence, define the gas phase shifting capability.The gap can also be fashioned out of parallel plates defining a planarinertance gap. In both the concentric ring configuration and theparallel plate configuration, the gap geometry is compact and light inweight. The compact gap design offers the capability of optimizing thephase shift for different applications by varying the gap size. Forin-operation adaptations, the gap can be made dynamic and preciselyexternally controlled when desired. A gap can be defined as between agap piston and gap housing. A variable gap is realized by moving thepiston within a gap housing. A motive means can be used to drive the gappiston toward or away from the housing to vary the thickness of the gap.That is, the position of the gap piston in relation to the gap housingdefines the gap thickness. The position of the gap can be variedmechanically by using the motive means that can be in the exemplar formsa motor using electrical power or thermal expander that is powered bydifferences in thermal contraction coefficients. Likewise, in acontrolled inertance tube device, the geometry of the tube can also bechanged, for example, by using a bellows tube.

Referring to all of the Figures, the gas phase shifting cryocoolerprovides more efficient cooling at each stage in a multistage displacercryocooler. The gas phase shifting in the displacer cryocooler can beachieved by installing a phase shifting device between the expansionspaces. For the displacer cryocooler, the exemplar gas phase shiftingdevice can be a flow resistance device, for example, a valve, asharp-edge orifice, or a porous medium. The gas phase shifting devicecan also be a flow inertia device, for example, a long capillary or aninertance gap. Likewise, the gas phase shifting pulse tube cryocoolerprovides more efficient cooling in a pulse tube cooler. The gas phaseshifting in the pulse tube cryocooler can be achieved by an inertancegap installed between the pulse tube and the reservoir. For the pulsetube cryocooler, the exemplar gas phase shifting device is a defined gappreferably having at least three degrees of design freedom foroptimizing the gas phase shifting. In both the displacer cryocooler andthe pulse tube cryocooler, the gas phase shifting device can beexternally controlled such as by using a controlled valve or acontrolled inertance gap changing the gas shifting characteristics forimproved cooler performance.

In a pulse tube cryocooler, an inertance gap is placed between the warmend of the pulse tube and the surge buffer reservoir. Exemplar gapsinclude concentric ring inertance gaps and parallel plate inertancegaps. The inertance gap is defined as a geometry with the thickness ofthe opening much smaller than the width or length of the opening. Assuch, the inertance gap provides three variables consisting of athickness Sg, a length Lg, and a width Wg of the gap having a volume Vg.The analogous LRC equations become L=L_(g)/(W_(g)S_(g)),R=12L_(g)η/ρW_(g)S³), and C=Mv/(γRT). Relating the inertance gapparameters to the inertance tube parameters, Lg=(4Wt/π) (S_(g)/d_(t)²)L_(t). The length of the inertance gap Lg is orders of magnitude(S_(g)/d_(t) ²) smaller than the length of the inertance tube L_(t). Theperformance of the inertance gap pulse tube is more efficient than thatof the inertance tube pulse tube at high powers, and is slightly lessefficient at low powers. Moreover, the design of the inertance gap canbe more compact, such as a few inches in length, compared to that of theinertance tube that can be a few meters in length.

The performance of a cryocooler can be predicted using commercialcryocooler software tools. For a two-stage displacer cryocooler, thefirst stage heat load can be plotted as a function of the first stagetemperature and the second stage heat load plotted as a function of thesecond stage temperature. In both stages, the cryocooler with the phaseshifting device offers a wider operating temperature range and a highercooling capacity.

The gas phase shifting cryocooler provides improved cooler performance.The addition of a control valve or orifice or controlled inertance tubeor gap between expansion volumes in a displacer cryocooler, or theplacement of an inertance gap in a pulse tube cryocooler, can provideimproved cooler performance. The gap design substantially decreases thelength of the phase shifting device reducing packaging constraints andthe potential for vibration failures. The performance of the inertancegap in the pulse tube cooler is improved at high powers. By furtheroptimizing the performance with a different geometry of the inertancegap, the gas phase shifting cryocooler can approach or surpass theperformance of an inertance tube pulse tube cooler. The controlled gasphase shifting cryocooler enables real-time optimization of theperformance of the pulse tube subject to different operating conditionsby varying the gap size remotely through a thermal or electromechanicalmeans. It is more practical to vary the inertance gap size rather thanthe dimensions of a long inertance tube. Instead of setting thedimensions of a valve or orifice or the inertance gap at discrete valuesthroughout the entire thermodynamic cycle, the dimensions of thecontrolled gas phase shifting device can also be varied within thethermodynamic cycle, resulting in a time variant phase shifting device.

A pulse tube with a compact inertance gap, which replaces the longinertance tube offers comparable performance, but is much more compact,easier to package, without any vibration failures while offeringreal-time performance optimization capability. Various gaps designed canbe used in a pulse tube cryocooler between the pulse tube and thereservoir. For example tapered, series, and parallel gaps could be usedin various configurations. Various gas phase-shifting devices, such as acontrolled valve or orifice, or inertance gap or tube can be usedbetween expander volumes in displacer cryocooler. The preferred formsare cryocoolers and heat pumps. Those skilled in the art can makeenhancements, improvements, and modifications to the invention, andthese enhancements, improvements, and modifications may nonetheless fallwithin the spirit and scope of the following claims.

1. A cryocooler a first volume and a second volume, the cryocoolercomprising, a first stage defining a first volume for containing gas ata first temperature and first pressure, and a second stage defining asecond volume for containing the gas at a second temperature and secondpressure, and a gas phase shifting device between the first volume andthe second volume, the gas phase shifting device providing a pressureand temperature phase difference between the first volume and the secondvolume, wherein, the cryocooler is a displacer cryocooler, the firststage is a moving first heat exchanger, and the second stage is a movingsecond heat exchanger.
 2. The cryocooler of claim 1 wherein, thecryocooler is selected from a group consisting of coolers, heaters, andheat pumps.
 3. The cryocooler of claim 1 wherein, gas mass flow is inphase with the gas pressure in the first volume and the second volume.4. The cryocooler of claim 1 wherein, the cryocooler comprises acompressor, the moving first heat exchanger and the second stage heatexchanger are portions of a porous piston driving gas pressure and massflow from the compressor.
 5. The cryocooler of claim 1 wherein, thecryocooler comprises a controller, and the gas phase shifting device isa flow impedance device.
 6. The cryocooler of claim 1 wherein, the gasphase shifting device is a valve.
 7. The cryocooler of claim 1 wherein,the gas phase shifting device is a flow impedance device selected fromthe group consisting of valves, sharp edge orifices, and porous mediums.8. The cryocooler of claim 1 wherein, the gas phase shifting device is aflow impedance device, and cooling of gas from the first volume to thesecond volume is controlled by flow impedance of the gas phase shiftingdevice for controlling mass flow and pressure waves between the firstvolume and the second volume.
 9. The cryocooler of claim 1 wherein, thegas phase shifting device is an inertance gap.
 10. The cryocooler ofclaim 1 wherein, the gas phase shifting device is a flow inertancedevice selected from the group consisting of tubes and gaps.
 11. Thecryocooler of claim 1 wherein, the gas phase shifting device is a flowinertance device, and cooling of gas from the first volume to the secondvolume is controlled by flow inertia of the gas phase shifting devicefor controlling mass flow and pressure waves between the first volumeand the second volume.